Have seen a couple of Newells that were 100 AMP service, that is with two 50 amp pigtails.. However, they were wired to accommodate standard 50 amp service should two 50 amp services not be available.
To answer the OP's question. You will likely seldom see a park with two 50 amp services at the site. It is much too expensive to run a second series of 50 amp services that will likely never, ever be used. Even if a park wired only one or two sites, they would likely to already be renting them to regular 50 amp rigs. No way would they consider keeping such a site vacant just hoping a white elephant showed up. ( it would have to be a very large site since any 100 Amp rig would be a 45 foot bus and all those are towing something). And no way would most parks allow running your generator. Unless you are in the $1,000,000 plus market, you aren't going to find a 100 amp RV. I will leave the gobblety-goop about 50 Amp service delivering 100 amps at 120 volts to others.